1. Field of the Invention
The present invention generally relates to a wafer-processing apparatus provided with a wafer alignment device, and particularly to a wafer handling device compensating for a deviation of a wafer thereon.
2. Description of the Related Art
In recent years, the number of wafers processed per unit time (=throughput) has become a matter of keen interest in connection with semiconductor manufacturing apparatuses used in the production of semiconductors. To improve the throughput, various ideas have been experimented on semiconductor manufacturing apparatuses, such as increasing the number of wafer processing chambers, providing multiple arms on the wafer handling device, and raising the wafer loading speed of the wafer handling device.
However, one major problem remains unaddressed, which is the deviation of wafers on the wafer handling device caused by higher wafer loading speeds. To be specific, when a wafer is placed in a position deviating from the specified position in the wafer processing chamber, various process problems will occur such as drop in the uniformity of film. Also, the wafer may drop into the wafer storage chamber during the process of returning the wafer to the wafer storage chamber, or the wafer may hit the chamber and get damaged.
A method generally known to solve this problem of wafer deviation is to place one or multiple photosensors at a position or positions forward in the wafer loading destination to measure a period during which light received by the photosensor is blocked, or the number of pulses from the encoder of the handling device, when the wafer handling device loads the wafer to the target destination, and compare the measured value against the pre-registered value corresponding to the correct wafer position, in order to detect a deviation of the wafer position and correct the wafer position.
Also adopted by the method explained above, digital sampling of output signals is a general method used to detect the sensor output while the wafer handling device is operating. Specifically, this is the method whereby signal values are converted into digital values in a chronological order and the converted digital data series is used to obtain the maximum value or minimum value. In recent years, signal sampling and data analysis are conducted using systems such as the EES (Equipment Engineering System).
However, high-speed operation of the wafer handling device is difficult to grasp when such sampling method is used. At the sampling frequency of 100 ms, which is a general sampling frequency used by EES, it is virtually impossible to follow the arm operation of the wafer handling device. If the operating speed of the wafer handling device is dropped to address this problem, the throughput of the semiconductor manufacturing apparatus will drop. If the sampling frequency is raised, on the other hand, the amount of data will increase and a storage device to save the data will become necessary. This will inevitably lead to a higher cost.
Also, deviation in the extending direction of the arm of the wafer handling device cannot be detected from the sensor output alone. To detect a deviation in the arm extending direction, the handling device controller must have a built-in wafer deviation detection part designed exclusively to monitor the operation of the wafer handling device itself and also count the number of pulses from the encoder of the handling device over a period during which light received by the sensor is blocked. Use of such detection part will present issues such as detection accuracy and cost increase.